1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) display apparatus and a method of manufacturing the organic EL display apparatus.
2. Description of the Related Art
In recent years, organic EL display apparatuses using organic EL devices made of light emitting materials are actively developed and studied as display devices having advantages such as a high-speed response and a wide viewing angle.
When an organic EL display apparatus including a large number of organic EL devices is to be driven by an active matrix circuit, it is necessary to connect each of the organic EL devices (pixels) with a set of thin film transistors (TFTs) for controlling a current flowing into each of the pixels.
In the active matrix organic EL display apparatus, minute transistors and capacitors are arranged above a substrate. Therefore, in order to improve an aperture ratio, it is desirable to employ a so-called top emission type in which light emitted from each of the pixels is extracted from the opposite side to the substrate as illustrated in FIG. 3.
A conventional active matrix top emission organic EL display apparatus will be described with reference to FIGS. 3 and 4.
Each pixel includes TFTs and an organic EL device which are stacked above a glass substrate 500.
A TFT portion 501 for driving the organic EL device is formed on the glass substrate 500. FIG. 3 illustrates a source region 510, a poly-Si layer 511, a drain region 512, a gate insulating film 513, a gate electrode 514, and an interlayer insulating film 515.
The TFT portion 501 is covered with an inorganic insulating film 517 and further covered with a planarization film 518 to planarize a surface of the glass substrate 500. A reflecting electrode (first electrode) 520 is formed on the planarization film 518.
The reflecting electrode 520 is formed for each pixel by patterning. The reflecting electrode 520 is electrically connected with a drain electrode 516 of one of TFTs included in the TFT portion 501 through a contact hole formed in the inorganic insulating film 517 and the planarization film 518.
A pixel isolation film 530 is an insulating film provided between adjacent pixels and arranged to cover a circumference portion of the reflecting electrode 520.
An organic layer 525 is formed on the reflecting electrode (first electrode) 520 serving as an anode. The organic layer 525 includes a hole transporting layer 523, a light emitting layer 522, and an electron transporting layer 524. A transparent electrode (second electrode) 521 serving as a cathode (common electrode) is formed on the organic layer 525.
In order to protect the organic EL device from moisture, a sealing glass material 540 is bonded to the resultant glass substrate 500 by a UV curable epoxy resin. A gap portion between the resultant glass substrate 500 and the sealing glass material 540 is filled with an inert gas 541.
As described above, in the case of the top emission type in which light is extracted from the opposite side to the substrate, a thin film made of a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO) is used for the second electrode. However, the transparent conductive material has a higher resistance than a resistance of a metal material.
Therefore, voltage drop is more likely to occur in the second electrode to apply different voltages to respective organic EL devices provided on a display surface. Thus, there is a problem that reduction in display performance due to a voltage gradient, such as reduction in light emission intensity at the central region of the display surface occurs.
In order to suppress the voltage gradient, it is desirable to form a low-resistance auxiliary wiring. In order to ensure an aperture of each of pixels, it is necessary to form the auxiliary wiring on a non-display region such as a region between pixels.
When the low-resistance auxiliary wiring is to be formed after the formation of the organic layer, an organic material of the organic layer deteriorates with water, an organic solvent, or ultraviolet rays. Therefore, it is difficult to pattern the formed auxiliary wiring by photolithography, so it is necessary to pattern the auxiliary wiring using a metal mask during the formation thereof.
When a film is to be patterned using a metal mask during the formation thereof by a vacuum vapor deposition method with a low-resistance material such as a metal material, it is difficult to maintain a constant distance between a substrate and the metal mask and to maintain high patterning precision because the metal material has a high evaporation temperature and the metal mask expands by radiation heat. In particular, in the case of a high-definition display panel, it is further difficult to perform patterning because a pixel interval is small.
Therefore, measures of providing the auxiliary wiring (electrically connected with the second electrode) between pixels before the formation of the organic layer have been proposed (Japanese Patent Application Laid-Open Nos. 2001-195008, 2002-318553, and 2001-230086).
Japanese Patent Application Laid-Open No. 2001-230086 discloses that an auxiliary electrode which includes an upper auxiliary electrode and a lower auxiliary electrode and has an overhanging cross-sectional shape is formed before the formation of the organic layer (see FIGS. 13 to 16) in Japanese Patent Application Laid-Open No. 2001-230086. The auxiliary electrode can be reliably electrically connected with an upper electrode (corresponding to the second electrode) through a portion located under an overhung upper portion of the auxiliary electrode.
The organic layer is made of an organic semiconductor material whose mobility is approximately 10−3 cm2/V·s to 10−6 cm2/V·s and thus whose resistance is extremely high. When the organic layer is located between the second electrode and the auxiliary wiring provided between pixels, it is difficult to electrically connect the auxiliary wiring with the second electrode. Therefore, in the case of the structure disclosed in Japanese Patent Application Laid-Open No. 2001-195008 or 2002-318553, as illustrated in FIG. 2, it is necessary to pattern the entire organic layer for the organic EL device so as to form a region in which no organic layer is provided on the auxiliary wiring.
In order to pattern the entire organic layer for organic EL devices so as to form a region in which no organic layer is provided on the auxiliary wiring, the required number of steps including alignment is equal to the number of organic layers. An apparatus cost required in the case where the entire organic layer is to be patterned is higher than an apparatus cost required in the case where only a part of the organic layer is to be patterned, such as the case where only the light emitting layer is to be patterned for each pixel. In addition, there are problems such as reduction in use efficiency of an expensive organic material used for the organic EL device, which is caused by lengthening a tact time required for film formation, and reduction in yield due to patterning deviation in a patterning step.
In the structure disclosed in Japanese Patent Application Laid-Open No. 2001-230086, even when the organic layer is not patterned for each pixel, a region in which the auxiliary electrode is exposed can be obtained without being covered with the organic layer. The region in which the auxiliary electrode is exposed corresponds to a rear surface of the upper auxiliary electrode and a side surface of the lower auxiliary electrode. Therefore, in order to bring the second electrode (upper electrode) to contact with the region, it is necessary to form the second electrode at a thickness equal to or larger than a required thickness. However, in the case of the organic EL display apparatus using the organic EL device of the top emission type in which light is extracted from the opposite side to the substrate, it is difficult to obtain high light extraction efficiency due to reduction in light transmittance.